Magnetic resonance imaging (MRI) devices require a highly intense, uniform magnetic field within a volume of sufficient size to allow a patient to be placed in the uniform field. Placing the patient into the field of the MRI system allows observation of living tissue in the patient This is valuable for medical diagnostic purposes, and is very instrumental in the early detection of cancerous tissues, tumors, and the like.
Different types of magnet systems have been proposed and utilized in an attempt to obtain a highly uniform field. Resistive magnet and superconducting magnet systems are two types. Permanent magnet systems are a third type, and appear to offer cost-effective advantages for MRI systems. One general design of a permanent magnet system has been proposed by W. Oldendorf in his article "Nuclear Magnetic Resonance and Correlative Imaging Modalities", Society of Nuclear Medicine, pp. 45-54, 1983. It utilizes an external frame of iron supporting two opposed permanent magnets which are oppositely charged, in which the pole faces have topographies intended to enlarge the useful uniform region of flux into which the patient may be placed for MRI analysis Another structure is that involving substantially parallel flat plates coupled together by a plurality of rod-like yoke portions, such as disclosed in U.S. Pat. No. 4,672,346 to Miyamoto. Another type of permanent magnet MRI system is manufactured by the same assignee of the present invention which utilizes a pair of oppositely charged flat parallel pole faces and attendant structure for providing a magnetic field uniformity in the air gap therebetween of forty (40) parts per million over a thirty (30) centimeter diameter spherical volume having central magnetic field strength of from six one hundredths (0.06) tesla to three tenths (0.3) tesla.
In permanent magnets currently being used in MRI systems, there are certain essential components One such essential component of MRI hardware is the gradient coil which produces localized linear gradients in the x, y, and z direction within the air gap of the MRI device. These local linear gradients are superimposed onto the homogeneous magnetic field within the air gap to provide location information. These gradient coils are utilized to ultimately provide spatial identification within the sphere. The gradient coils are parallel, and typically located a few centimeters from the large flat ferromagnetic pole faces utilized in the permanent magnet MRI devices.
Unfortunately, when these gradient coils are pulsed during the utilization of the MRI system, the pole faces of the MRI device become additionally magnetized so that a substantial remnant gradient in the x, y or z direction exists. Whenever a closed electrical path is exposed to a time varying magnetic field, an AC electrical eddy current is induced in this path. These eddy currents generally have a relatively short time constant and act in a way to oppose the changing magnetic field. It is known that eddy currents flowing in ferromagnetic materials can cause magnetic hysteresis Thus, such remnant gradients add unwanted gradients to the field so as to degrade the homogeneity of the magnetic field in the air gap and further impair image quality Furthermore, it is well known that minimizing eddy current signature, i.e. reducing the time constant and amplitude associated with the transient response, results in beneficial performance of the system.
The present invention thus recognizes the need to minimize such AC eddy current induced hysteresis, and the need to maintain the homogeneous and uniform magnetic field. The present invention recognizes that by providing a shielding layer of nonferromagnetic conducting material on the pole faces, eddy currents are transferred from the pole face to the nonferromagnetic layer, thus minimizing any induced hysteresis. A nonferromagnetic material, such as aluminum (Al) or copper (Cu) plates, is one such nonferromagnetic material which conducts electricity. The present invention further recognizes that hysteresis can be minimized by providing a pole face with a structure which substantially reduces the path of the eddy current. Unwanted eddy current effects can be minimized so that the time constant is shortened considerably, on the order of one millisecond or less. Moreover, the amplitude of the eddy current, voltage and current signature are reduced to a very small value, thus not introducing unwanted remnant gradients into the MRI system.
Accordingly, it is an object of the present invention to provide a method and apparatus for minimizing the hysteresis induced in the pole face of a permanent magnet-type MRI system. It is a further object of the present invention to provide a method and apparatus for achieving a highly uniform magnetic field suitable for use in MRI. It is yet another object of the present invention to provide a system and apparatus for minimizing hysteresis while maintaining uniformity in the magnetic field of the air gap of an MRI system. Still another object of the present invention is to provide a method and apparatus for minimizing hysteresis in the MRI system environment which is cost-effective and easily utilized and manufactured.